This invention relates to rotating field machines in general and more particularly to a rotating field machine fed by a frequency converter.
A rotating field machine drive with a frequency converter, a rotating field machine fed by the converter, two a-c voltage integrators, and a converter control connected to the outputs of the a-c voltage integrators is known in the art. Each a-c voltage integrator comprises an integrator having a component of the EMF vector of the machine, made up of the stator currents and stator voltages of the machine, fed to its input and has, as an output, a corresponding component of the magnetic flux vector of the machine. It further includes a feedback line connecting the integrator output with the integrator input, as well as a zero controller in the feedback line. Nominal values for the independent control of the stator current component parallel to the flux vector and the component normal thereto are introduced in the converter control.
In an asynchronous machine, the component of the stator current vector, which is formed by vectorial addition from the currents flowing in the stator windings in consideration of the winding axes, parallel to the axis of the magnetic flux (magnetization current) determines the magnetic flux of the machine, while the component normal thereto (active current) influences the torque of the machine. Now if it is possible to control the converter feeding the stator windings in such a way that the component parallel to the field and hence the intensity of the magnetic field induced in the machine remains constant, the torque of the machine can be controlled through the component of the stator current normal to the field. Such a field oriented regulation excels by its great accuracy and clear lay-out combined with optimum utilization of the converter and of the machine. Since the nominal value of the stator current is set with respect to the two field oriented coordinates or at least by the magnitude and angular position of the stator current vector with respect to the flux vector, the position of the flux vector with respect to the stator current windings, i.e., in a reference system fixed in space relative to the stator, must be known for the control of the converter.
The situation is similar in a synchronous machine, although here, in addition to the field parallel component of the stator current, the field parallel component of the exciter current which also contributes to the formation of the magnetic flux must be taken into consideration. Hereinafter the magnetizing current i.sub..phi.1 in a sychronous machine, therefore, is always understood to be the field parallel component of the vector sum of a stator current and exciter current.
The position of the magnetic flux vector can be determined by Hall probes, although such is generally avoided, in particular because of the space requirement and greater trouble proneness of the Hall probes under the prevailing conditions. In the above described arrangement, which is disclosed in German Pat. No. 28 33 542, therefore, the position of the magnetic flux vector and its magnitude is formed by calculating two components of the magnetic flux vector in a stator related coordinate system from input quantities of the rotating field machine using a computing model.
The computing model used may be termed a voltage model, since the Y-voltage at a machine lead is, after subtraction of the ohmic voltage drop, the EMF occurring at this lead, from which the contribution to the flux of the machine supplied by the respective stator winding can be formed by integration. By addition of a signal proportional to the respective machine current, the leakage inductance of the machine can then also be taken into account. Thus the two a-c voltage integrators of the known circuit, which are assigned to the voltages and currents in two different machine leads, have, as output quantities, two components which determine the flux vector in a coordinate system established by the winding axes of the stator windings. Instead of the direct input of the Y-voltage and of the current of a machine lead, analogously to the stator current vector, by corresponding vectorial addition of the stator voltages, a voltage vector in a stator-related coordinate system, for example a Cartesian one, the components of which together with the corresponding components of the current vector are used as input quantities for the a-c voltage integrators may be formed. The flux vector is then also calculated in the components of this coordinate system.
Circuit arrangements as described in German Pat. No. 28 33 593 and as provided in the method according to the above-mentioned German Pat. No. 28 33 542 are especially suitable as a-c integrators for such a field oriented control using a voltage model.
To avoid drifting of the integrators, however, the integrator zero must be maintained constant by a zero control. To this end according to German Pat. No. 28 33 593 a zero controller consisting of a P-controller and an I-controller is used in the integrator feedback line, the intervention of which is weighted as a function of the frequency. Together with the zero drift of the integrator, however, slow changes of the flux which are to be obtained at the integrator output and which correspond to low operating frequencies are also suppressed. Under stationary operation, moreover, the arrangement produces an angle error which, especially at low frequencies, also leads to a disturbing misorientation if, in the operation of the rotating field machine, the nominal values of the current vectors to be fed are oriented to the flux vector which is determined. The good dynamics of the current model is offset, therefore, by a mis-orientation in stationary operation which may lead to disturbances especially at low frequencies. To this must be added that, even at higher operating frequencies, there occur, for harmonics and subharmonics caused by the static converter, values for the damping and for the angle error which differ from the value tuned to the operating frequency and which may lead to the result that these oscillations are no longer sufficiently damped.
A model value for the flux actually occurring in the rotating field machine can, however, also be determined by another computing model circuit ("current model"), to the inputs of which only input voltages which correspond to the stator current, the magnetization current, and the rotor position are supplied. This computing model circuit simulates the processes which occur in the rotating field machine, and which lead to the development of the flux using corresponding electronic computing units; it is laid out differently depending on the type of rotating field machines used (synchronous or asynchronous machine). The paper "Control methods for rotating field machines" read at the Bildungswerk (Educational Institution) of the "Verein deutscher Ingenieure" (Association of German Engineers), the manuscript of which is sold by the VDI-Bildungswerk, Dusseldorf, under Order No. BW 3232, analyzes the structure of asynchronous and of synchronous machines, respectively. For all further discussions the nomenclature introduced therein is used, in which the indices .phi.1 and .phi.2 indicate the field parallel and field normal components of a vector; the indices .alpha. and .beta., the vector components in a Cartesian reference system fixed in space; and the index s, a quantity occurring in the stator.
In FIG. 5 on page 16 a frequency converter U for an asynchronous machine is shown, to the control of which there are supplied, on the left side, the nominal values for the stator current component parallel to the flux (magnetization current i.sup.s*.sub..phi.1) and for the stator current component normal to the flux (i.sup.s*.sub..phi.2). The converter feeds the asynchronous machine shown at the right in an equivalent structure with a stator current which, in a stator related coordinate system, has the magnitude i.sup.s and the angle .epsilon..sup.s.sub.3. For the control of an asynchronous machine there is indicated, at the left, a computing model circuit which determines, from the field-oriented nominal current values, the slip frequency, and therefrom, by means of the rotor position which is introduced as angle .lambda..sub.s of the rotor axis with respect to an axis of the stator-related coordinate system, the angle of the model flux vector in the stator related coordinate system. By a dynamics element (delay element) a model value for the actual value of the magnetization current and hence for the amount of the flux occurring is formed from the magnetization current component i.sup.s*.sub..phi.1 of the stator current nominal value fed in. This model circuit, therefore, furnishes a model value, calculated only from the stator currents, for the magnitude and the angle of the flux in stator related polar coordinates, which, if necessary, can be converted to the components of the model flux with respect to Cartesian stator fixed axes.
Similarly, in FIG. 10 on page 19, an arrangement with a synchronous machine is indicated, where there is also supplied to the converter U at the left portion, next to the nominal values i.sup.s*.sub..phi.1 and i.sup.s*.sub..phi.2, the nominal value i.sup.e*.sub.100 1 for the exciter current component parallel to the flux. In the case of the synchronous machine there exists between i.sup.s.sub..phi.1, i.sup.e.sub..phi.1 and the magnetizing current i.sub..phi.1 the relationship i.sup.e.sub..phi.1 =i.sub..phi.1 -i.sup.s.sub..phi.1. The converter U, and its control, feed the exciter winding with an exciter current i.sup.e.
In the current model it is a disadvantage that the model parameters must be adjusted very exactly to the machine parameters, and therefore a temperature related change of the rotor resistance, for example, will lead to errors in the determination of the model flux in stationary as well as in dynamic processes. Since, however, the flux determination at low frequencies is still more accurate, despite the sometimes unsatisfactory dynamics, than in the voltage model, the current model is preferred over the voltage model if the rotating field machine is to be operated in the low speed range.
It is the object of the present invention to provide a circuit arrangement which forms, both in the low and high speed ranges, a voltage signal which is proportional to a flux component of the flux occurring in a rotating field machine and which has a phase and amplitude error as small as possible independent of the machine frequency. At the same time the good properties of the device according to German Patent DE-PS No. 28 33 593 (voltage model) in the upper speed range are to be preserved or even improved.